Modern drive units include an internal combustion engine, for example a four-stroke gasoline or Diesel engine, in which the combustion air is compressed to a pressure greater than atmospheric pressure to improve performance. Depending on the variant, the drive unit includes one or more, typically two compression stages. Drive units with one compression stage are usually equipped with a turbocharger device or a compressor device including a compressor. Drive units with two compression stages include either two turbocharger devices or a turbocharger device and a compressor device. The compressor device may include a rotary compressor in the form of a Roots-type supercharger, for example. Whereas the turbocharger device is driven by the exhaust gas enthalpy of the exhaust gases in the internal combustion engine, the compressor device is driven either by the internal combustion engine itself or by means of an electric motor. Internal combustion engines in which the combustion air is compressed are often also called “forced induction” internal combustion engines.
If the compressor device is driven by the internal combustion engine itself, the compression performance provided by the compressor increases with the speed of the internal combustion engine. In this case, a coupling device may be provided, with which the compressor device can be coupled to the internal combustion engine or uncoupled therefrom depending on the speed of the internal combustion engine, At low engine speeds, the compressor device delivers only a fraction of its compression capability, as a result of which the torque output from the internal combustion engine is low at low engine speeds. Since the turbocharger device's compression performance depends on the pressure in the discharge line, which also increases with the speed of the engine, the output even from internal combustion engines with two-stage supercharging is low at low engine speeds, which is particularly noticeable as a negative phenomenon during an acceleration operation. Accordingly, this behavior is referred to as poor response behavior of the internal combustion engine.
Particularly in high-performance engines, the compressor device must have relatively large dimensions in order to cover a sufficiently wide operating area. However, as the compressor becomes larger, a not inconsiderable portion of the torque delivered by the internal combustion engine is required to drive the compressors, which not only exacerbates the drawback of weak torque at low engine speeds described previously, but also increases fuel consumption.
If the compressor device is driven by an electric motor, the speed of the compressor can be set and altered independently of the speed of the internal combustion engine. In this case, it is not absolutely necessary to arrange a coupling device between the compressor and the electric motor. When an electric motor is used, the response behavior of the internal combustion engine can he improved, but then the electric motor must supply a certain output to bring the compressor up to the optimal speed, thus entailing increased consumption of electrical energy. Since the electrical energy is provided by a generator, which is driven by the internal combustion engine, the increased consumption of electrical energy also causes increased fuel consumption.
Particularly given the ongoing efforts to lower CO2 emissions, increased fuel consumption by internal combustion engines with one or two compression stages is undesirable.